In recent years, with the rapid progress of research concerning the structures and functions of sugar chains, research efforts have been undertaken to develop applications of oligosaccharides, glycolipids, glycoproteins, and similar materials having physiological activities in the fields of drugs and functional materials. Among sugar chains, a sialic-acid-containing sugar chain having N-acetylneuraminic acid (NeuAc) at an end thereof is known to play an important role as a receptor in, for example, cell adhesion or viral infection.
Generally, the sialic-acid-containing sugar chain is synthesized by use of sialyltransferase as a catalyst. Sialyltransferase is an enzyme which catalyzes the transfer of sialic acid from cytidine-5′-monophosphate-N-acetylneuraminic acid (CMP-NeuAc), which serves as a sugar donor, to a sugar chain serving as an acceptor.
Basically, CMP-NeuAc is synthesized from cytidine 5′-triphosphate (5′-CTP) and neuraminic acid (NeuAc) serving as substrates by use of CMP-NeuAc synthase serving as a catalyst.
However, CMP-NeuAc, which is employed as a sugar donor, is very unstable and is difficult to prepare in large amounts. Therefore, CMP-NeuAc is very expensive and has been provided in such small amounts that would be good for reagent uses. Meanwhile, the CMP-NeuAc products currently in supply have a low purity (typically, a purity of 90% or thereabouts as determined by HPLC), since a highly pure product having a purity of 95% or more has conventionally been difficult to produce. Such a low purity inevitably renders CMP-NeuAc inappropriate as a raw material for producing a sialic-acid-containing sugar chain or a similar product.
According to a conventionally favored CMP-NeuAc purification process, cytidine 5′-monophosphate (5′-CMP), cytidine 5′-diphosphate (5′-CDP), and 5′-CTP—which are generated through degradation during reaction for producing CMP-NeuAc or during subsequent purification steps, or remain in the synthesis reaction mixture, and which are very difficult to separate from CMP-NeuAc—are all converted into cytidine compounds through removal of phosphate moieties from 5′-CMP, 5′-CDP, and 5′-CTP, which coexist with CMP-NeuAc, by use of calf-derived alkaline phosphatase (CIAP); and subsequently CMP-NeuAc is separated from the resultant cytidine compounds through chromatography treatment, such as ion-exchange chromatography or gel filtration chromatography (Patent Documents 1 to 3 and Non-Patent Document 1).    Patent Document 1: JP-A-H5-276973    Patent Document 2: JP-B-H5-73391    Patent Document 3: JP-A-H8-73480    Non-Patent Document 1: J. Am. Chem. Soc., 110, 7159-7163 (1988)